Synthesis of Safety for Traffic Operations - Transports Canada

Synthesis of Safety for Traffic Operations March 2003
TABLE 3.45: Safety Impacts of Left-turn Phasing Correlated with Traffic Volume
and the Presence of a Left-turn Lane
Volume LTL
Crashes
Before After
Change
13520 N 2 2 No change
14375 N 4 4 No change
14589 N 2 1 Decrease
31797 N 8 18 Increase
10691 Y 5 3 Decrease
12216 Y 5 2 Decrease
12270 Y 4 6 Increase
16384 Y 6 4 Decrease
22694 Y 3 7 Increase
23323 Y 5 7 Increase
25697 Y 11 11 No change
30539 Y 20 10 Decrease
36196 Y 10 12 Increase
Hummer et al (1991)
Hummer et al (1991) in developing guidelines for leading and lagging left-turn phasing in
Indianapolis examined crash data from 14 intersection approaches with lagging left-turn
phasing, and 15 approaches with leading left-turn phasing. Almost all of the study
locations were the intersection of a two-way street with a one-way street located in the
downtown area. All locations were fixed-time signals. Four years of crash data were
used to identify crashes involving a vehicle turning left from the approach of interest (i.e.,
target crashes). The target crashes were coupled with traffic volumes to determine
applicable crash rates for comparison. The results of the comparison are shown in Table
3.46.
TABLE 3.46: Crash Rates for Leading and Lagging Left-turn Phasing
Statistic Lagging Leading
No. of approaches 14 15
No. of Left-turn crashes 44 69
Crash rate per 10 6 left-turns 0.8 0.9
Crash rate per MVE 0.06 0.09
The results indicate that lagging left-turn phasing is slightly safer than leading phasing.
However, the relatively small sample size is insufficient to draw any definitive
conclusions. It should be noted that a more accurate determination of relative safety may
have been achieved had the crash rate been calculated using the product of left-turn
volume and opposing volume as the measure of exposure. Nonetheless, Hummer et al
Page 51